Continuous casting machines are used in the basic metal industry to continuously produce semifinished billets, slabs, and the like from molten metal in a one-step solidification process. Generally, molten metal from a tundish is continuously introduced into a water-cooled mold where initial solidification takes place in the form of a frozen metal skin surrounding a liquid core as the cast product continuously leaves the mold. The molten metal continues to solidify progressively inwardly in a secondary cooling zone where complete solidification of the cast product occurs in the spray cooling, roll cooling and radiation cooling zones located beyond the caster mold.
In order to have successful caster operation, a precise amount of solidification or skin growth must continuously occur. If too much heat is removed, surface cracks and internal defects may develop in the strand. If too little heat is removed, a breakout of molten metal will occur in the caster which may result in serious consequences to both personnel and facilities.
Therefore, it is important to continuously control the caster heat removal rate for preventing problems which can occur in the continuous casting operation. Early casters had only simple control of spray cooling water rates. The flow in each zone was usually set before each cast and remained constant. As more grades with greater temperature senstivity were cast a better means of cooling control was needed. A first known method for controlling the cooling of the cast product in a continuous casting involves controlling the flow in each zone as a direct function of casting speed. In this method, the cooling water rates are correct only when steel passing through a spray zone has traveled at a steady speed from the time it started at the meniscus. When speed changes occur, the strand will experience temperature perturbations until fresh steel at a new steady state fills the spray chamber. These temperature disturbances can cause surface and internal defects.
To overcome this deficiency, a second known method has been the control of flow as a function of the steel residence time in each zone. With residence time control, theoretically each element of steel entering the machine is cooled in the same way since time is the controlling factor. As speed changes, the entire cooling profile along the machine changes. For example, if the maximum speed is reduced by 50% then the new steel entering the machine is cooled in 50% of the spray distance. This known method of controlling the coolant water flow as a function of the steel residence or elapse time in each zone is described, for example, in British Patent Specification No. 1,302,040 published on Jan. 4, 1973 and in U.S. Pat. No. 4,463,795 to Chielens, et al. In the Chielens, et al. patent, there is described a predictive method of controlling cooling by making residence time calculations and water flow calculations using heat transfer curves. Although the residence time technique is an improvement over previous control methods, important deficiencies still exist. The residence time is a purely predictive control based on calculations, while correct cooling requires that flows follow the computing setpoints. This is not possible in many instances due to mechanical and operational caused problems including the process hardware limitations described below.
Also, there have been proposed the use of temperature controls for overcoming some of the below described deficiencies. In the U.S. Pat. No. 4,073,332 issued on Feb. 14, 1978 to A. T. Etienne, the temperature is measured along the surface of the strand in a series of secondary cooling zones, and the specific coolant flow rate to the zones is varied to maintain a desired thermal profile along the surface of the strand in relation with the casting speed and residence time in each cooling zone. One problem with this system is that the temperature measuring devices cannot be maintained and do not have sufficient capability to give reliable measurements because of the high temperatures and the water and steam environment.
Spray nozzles have a limited operating range and cannot be operated satisfactorily when the flow rate is less than 20 to 33% of the nozzle maximum. Cooling patterns deteriorate and strand cooling becomes irregular when low flow rates are required. Also, most casters have small diameter rolls in the upper part of the machine near the mold which are not internally cooled and require that spray water always be flowing to cool the support rolls and prevent thermal damage. Thus, the strand may be cooled by water when none is required, by a flow rate determined by the roll cooling requirement rather than the strand cooling requirement. Here, again, the predictive control would be bypassed. Finaly, since the spray chamber is divided into a finite number of zones, the residence time control can only supply correct cooling to a small part of a zone. These restrictions cause improper cooling of the strand during transient and abnormal operating conditions. In the situation of long stops or slowdowns for grade, width or tundish changes, which allow for increased productively, severe overcooling of the strand usually results with an attendant decrease in quality.